The Denver Mint produced a major Overpunched Mintmark (OMM) error on the 1944 D Lincoln Wheat Cent (Penny). The best way to identify this error is to note the "D" mintmark stamped on top of a subtle "S" mint mark (D/S OMM). The location of the "S" is depicted by the arrow in the example image above where part of the "S" is best seen above the "D".
One of the rarest and most intriguing US coins is the 1944-D Steel Lincoln Wheat Cent. Nearly all pennies from 1944 are supposed have transitioned from steel back to copper/bronze. However - similar to the rare 1943 copper cent - a small number of zinc-coated steel planchets were left over from 1943 and were used in 1944 during the transition to steel planchets. The 1944 steel cent was produced at all three mints. However - Only 7 Denver-minted 1944-D Steel Cents are known to exist - making it rarer than the Philadelphia variety but more common than the San Francisco variety. Most experts believe that there are still a few yet to be discovered!